One hundred years ago, in 1913, Niels Bohr published a trilogy of papers describing the atom in ways that we are still referring to today. These papers synthesized several previous ideas into one and presented the atom as a positive nucleus encircled by a cloud of electrons that stayed in very distinct orbits that were proportional to their energy (i.e. electrons in low orbitals were lower energy than those that inhabited higher orbitals).
This sounds like a simple concept, but it has some very important ideas nested in it. Probably most importantly is the idea that the orbitals are distinct. That is, there is no ‘between’ orbitals. Electrons are in one orbital or another, but never in between. Also, it takes energy input to raise an electron into a higher orbital and energy is released (as light) when an electron drops into a lower orbital. As a biologist, I find this most interesting and useful to think of when contemplating photosynthesis and considering how photons are absorbed by atoms in the reaction centers of chloroplasts. This is done by raising an electron to a higher … let’s say ‘energy level’. Once this happens, we have energy stored (at least for a while) in this electron. That energy can be used to do work, passed on to another atom or it can release the energy into the environment as light.
Here’s a good representation of Bohr’s model with the different energy levels / orbitals indicated by the dotted lines labeled n=1, n=2 and n=3 .
Until now, this model has been a good, workable theory that seemed to fit mathematically with what was observed indirectly. However, two really cool papers came out recently that have provided the first direct observations of atoms / molecules. In the first, Hydrogen atoms were observed using photoionization microscopy. This was done with a hydrogen at resting state (it’s lone electron in the lowest orbital), and in several higher energy states attained by providing energy to the atom using a laser. Below is a figure from the paper presented in Physical Review Letters 110, 213001 (2013). In each subsequent panel the electron can be seen in increasingly higher (distinct) orbitals.
Ok, I’m just going to come out and say it, ‘This is totally f’n cool.’ This means that Bohr’s totally theoretical model of a century ago has just been directly shown to be completely accurate.
But wait, there’s more.
We’ve been using Bohr’s model and others’ ideas to model how multiple atoms come together to form molecules. Again, these structures have always been imagined from indirect observation. But, in the May 30 Science, this too has been directly observed using non-contact atomic force microscopy. Here we can see atoms in a molecule as well as the covalent bonds between them.
Here’s to you Niels. Bang up work!! Not to mention Dimas G. de Oteyza1,2,*,Patrick Gorman3,*, Yen-Chia Chen1,4,*, Sebastian Wickenburg1,4,Alexander Riss1, Duncan J. Mowbray5,6, Grisha Etkin3, Zahra Pedramrazi1, Hsin-Zon Tsai1,Angel Rubio2,5,6, Michael F. Crommie1,4,†, and Felix R. Fischer3,4,† for imaging the covalent bonds.